FR2647901A1 - CONSTRAINED GAUGE WEIGHING DEVICE - Google Patents

Abstract

The loading cell (1) consists of a slightly flattened flexion bar (4) combined with a wide flat parallel secondary sheet (13) connecting a mobile upright (6) to a fixed upright (9). A plate (2) is firmly attached to the mobile upright (6) while a base plate (3) is firmly attached to the fixed upright (9). Strain gauges are distributed over the surface of the flexion bar (4) and are connected in a Wheatstone bridge for the measurement of the load.

Description

CONSTRAINED GAUGE WEIGHING DEVICE

  The present invention relates to load cells, used

  especially in the production of weighing scales such as a

  bathroom scale, food scale or other platform weighing device.

  Plate weighing devices generally consist of a base intended to rest on the ground, and a plate substantially parallel to this base. The tray is intended to receive the load to be weighed. Between the base and the plate is placed a load cell comprising strain gauges electrically connected according to a Wheatstone bridge and delivering an electrical signal proportional to the load applied to the plate. The load cell comprises an elastically deformable bending bar, arranged in a direction substantially perpendicular to the direction of the force to be measured, that is to say substantially parallel to the plate. A mobile upright connects the plate to a first end of the bending bar, the mobile upright being generally parallel to the direction of the force to be measured. A fixed upright, also generally parallel to the direction of the force to be measured, connects the base and the second end of the bending bar. The strain gauges are fixed on the surface of the bending bar, and are sensitive to the elongations and shortenings of said surface caused by the bending of said

  closed off. Such a structure is for example described in the document FR-

A-2 177 923.

  The difficulty of these load cells lies in the fact that the electrical signal that is obtained is sensitive to the position of the load on the plate. It can be seen that the bending characteristics of the bending bar vary on the one hand as a function of the longitudinal eccentricity of the load on the plate or position in the direction of the length of the bending bar, and on the other hand part according to the transverse eccentricity of the load on the plate or position in the direction perpendicular to the length of the bending bar. The longitudinal displacement of the load introduces a variation in the torque applied to the first end of the bending bar by the mobile upright along a transverse axis. The displacement of the load transversely on the plate introduces a variation in the torque applied to the first end of the bending bar by the movable upright along a longitudinal axis. These torsional stresses deform the bending bar and disturb the

  measurement of its bending by the strain gauges.

  In document FR-A-2 177 923, the effects of longitudinal displacement of the load on the plate are compensated for by placing several strain gauges in at least two zones offset longitudinally on the bending bar. The device remains however

  sensitive to the movement of the load in a transverse direction.

  We know load cells made in a single block and

  machined to make them insensitive to transverse eccentricities

  the charge. Such load cells are however very expensive because of the precise machining they require. These machining operations must be carried out by successive approximations, and depend on a large number of parameters making mass production illusory.

without recovery.

  Document WO-A-8 402 186 teaches to make such a load cell insensitive to transverse load eccentricities by using additional strain gauges sensitive to the torsion of the bending bar. This solution involves using additional gauges, which must be arranged very precisely on the bending bar, and requires subsequent adjustment of the electrical signals. Experience shows that this adjustment is most often insufficient, and requires precise machining of the bending bar. We are also led to multiply the strain gauges, which multiplies the connections to be made during assembly, multiplies

  the risk of breakdowns, and significantly increases the cost.

  Thus, all known solutions require adjustments during the manufacture of weighing devices, and these adjustments

  made in recovery significantly increase the cost of production.

  The object of the present invention is to produce a load cell with strain gauges, of simple design and structure so as to be low cost, and which, by construction, is practically insensitive to load eccentricities, both longitudinal

than transversal.

  According to the invention, these advantages are obtained without multiplying

the number of strain gauges.

  In addition, according to an advantageous variant of the invention, it is sought to obtain this insensitivity to load eccentricities while avoiding dispersing the strain gauges along the entire length of the bending bar. Such a dispersion complicates the electrical connection operations during assembly. Thus, the invention makes it possible to gather all the strain gauges in a single zone

of the bending bar.

  To achieve these and other objects, the load cell according to the invention comprises: - an elastically deformable bending bar arranged in a direction substantially perpendicular to the direction of the force to be measured, - a mobile upright secured to a first end of the bending bar and substantially parallel to the direction of the force to be measured, intended to be secured to a plate receiving the load to be measured, - a fixed upright secured to the second end of the bending bar and substantially parallel to the direction of the force to be measured, intended to be integral with a base, - strain gauges, fixed on the surface of the bending bar, and intended to be electrically connected to an electrical measurement circuit for supply an electrical signal as a function of the flexural deformation of the flexion bar under the action of the mechanical force applied between the fixed and movable uprights, - a blade is flat and wide, elastically deformable, arranged

  away from the bending bar in a plane substantially perpendicular to

  the direction of the force to be measured, fixed respectively at its first end to the mobile upright and at its second end to the fixed upright, so that the secondary blade absorbs the forces of

  torsion due to the eccentricity of the force to be measured.

  In applications to a weighing device with a receiving plate, the mobile upright is integral with a load receiving plate developing along a length L1 parallel to the bending bar and along a width L2 perpendicular to the bending bar; the secondary blade advantageously has a substantially constant width close to the width L2 of the receiving plate

dump.

  According to an advantageous embodiment, the strain gauges are arranged in a single zone of the bending bar, away from the central zone. In this case, the first respective ends of the bending bar and of the secondary blade are spaced from each other by a distance D1 greater than the distance D2 separating their second respective ends. In this way, the bending bar makes an angle A with the secondary blade, the value of which is adjusted by construction so as to compensate for the effects of

  longitudinal off-center effort.

  Other objects, features and advantages of this

  The invention will emerge from the following description of embodiments.

  tion particular, made in connection with the accompanying figures, among which: - Figure 1 is a perspective view of a load cell according to the invention associated with a load receiving plate and a base; - Figure 2 shows schematically a side view of a load cell according to a first embodiment of the invention; - Figure 3 shows the load cell of Figure 2 in top view, partially cut movable upright; - Figures 4 and 5 illustrate, on a larger scale, the constitution of the strain gauges of Figure 3; - Figure 6 shows schematically, in side view, a load cell according to a second embodiment of the invention; - Figure 7 shows a top view of the load cell of Figure 6, partially cut movable upright; - Figures 8 and 9 illustrate, on a larger scale, a possible arrangement of the strain gauges of Figure 7; - Figures 10 and 11 illustrate, on a larger scale, another possible arrangement of the strain gauges of Figure 7; - Figure 12 schematically illustrates, in side view, a load cell according to a third embodiment of the invention; - Figure 13 shows the load cell of Figure 12 in top view, partially cut movable upright; - Figure 14 shows, on a larger scale, the arrangement of the strain gauges of Figure 13; and - Figure 15 illustrates the electrical diagram of connection of the gauges of

constraint.

  In the embodiment shown in FIG. 1, the device of the invention comprises a load cell 1, a tray

2647901-

  load receptor 2 and a base 3.

  The load cell 1 comprises a bending bar 4, elastically deformable by bending, arranged in a direction substantially perpendicular to the direction 5 of the force to be measured. A mobile upright 6 is secured to a first end 7 of the bending bar 4, and is substantially parallel to the direction 5 of the force to be measured. The mobile upright 6 is integral with the load receiving plate 2 intended to receive the load 8 to be measured. A fixed upright 9 is integral with the second end 10 of the bending bar 4, and is substantially parallel to the direction 5 of the force to be measured. The fixed upright 9 is integral with the base 3. Strain gauges are fixed on the surface of the bending bar 4, and are electrically connected to an electrical measurement circuit, not shown in the figure, to provide an electrical signal which is a function of the bending deformation of the bending bar 4 under the action of the mechanical force produced by the weight of the load 8 placed on the plate 2. In the embodiment shown in FIG. 1, the strain gauges are arranged in two separate zones 11 and 12, respectively in the vicinity of the first end 7 and the second end 10 of the bending bar 4, at equal distances from the middle of the bar. Zones 11 and 12 are located on one of the flex bar 4 faces which are

parallel to the plate 2.

  A secondary blade 13, of flat and wide shape as shown in the figure, is disposed away from the bending bar 4 in a plane substantially perpendicular to the direction 5 of the force to be measured. The secondary blade 13 is elastically deformable by bending, and is fixed respectively at its first end 14 to the movable upright 6 and at its second end 15 to the fixed upright 9. The secondary blade 13 is located substantially parallel to the flexion bar 4, which it is separated by a distance D in the

direction 5 of the effort to be measured.

  When the load 8 is moved from its central position, shown in solid lines in FIG. 1, to its laterally off-center positions 16 and 17, the load 8 produces a torsional torque of longitudinal axis on the load cell 1. The presence of the secondary blade 13 very substantially reduces the torsion angle of the load cell 1 produced by the torsional torque of the load 8 along the longitudinal axis. As a result, the presence of the secondary blade 13 makes the effect of the transverse offsets of the load 8 on the plate 2 practically negligible. To ensure good recovery of the torsional forces, it is advantageous to use a secondary blade 13 whose width L is as large as possible. In practice, for an application to a balance comprising a plate 2 of length L1 and of width L2, the length L ′ being the dimension parallel to the length of the bending bar 4, we use

  advantageously a secondary blade 13 whose width L is appreciable-

  constant and close to the width L2 of the receiving plate of

charge 2.

  Furthermore, to obtain a good sensitivity of the measurement of the force, it is advantageously possible to use a bending bar 4 whose width is relatively reduced and whose thickness is relatively large. Under these conditions, the surface length or width variations undergone by the strain gauges placed on the upper surface or the lower surface of the bending bar are maximized. At the same time, it is necessary to avoid reducing the bending of the bar for a given load, in order to maintain good sensitivity. It is therefore advantageous to use a secondary blade 13 whose stiffness is low compared to the stiffness of the bending bar 4, so that the secondary blade 13 does not decrease

  substantially the bending of the bending bar 4 for a given load.

  For this, in the direction of the flexions produced by the force to be measured such as the force of the load 8, the stiffness of the flexion bar 4 is chosen to be much greater than the stiffness of the secondary blade 13. This difference is obtained for example by using a bending bar 4 whose section has a relatively large thickness relative to aa width, while the secondary blade 13 is flat and wide. We can accentuate the difference by making the bending bar 4 and the secondary blade 13 in different materials chosen from

appropriately.

  The reduction or compensation for disturbances introduced by the decentralization of the load 8 in the longitudinal direction can be ensured in different ways. When the load 8 is moved longitudinally, for example, -r take its position 18 or its position 19, the presence of the secondary blade 13 very significantly reduces the deformations induced on the bending bar 4, compared to a load cell 1 which would have only one bending bar. Longitudinal decentering, however, introduces axial compression or tensile stresses on the bending bar 4 and the secondary blade 13, and a slight deformation of the bending bar 4 can be observed, deformation inducing a

  variation of the measured electrical signal.

  To compensate for such variations due to longitudinal decentering, it is possible to use a known technique consisting in placing the strain gauges in at least two zones 11 and 12 offset longitudinally along the bending bar 4 and at equal distances from the middle of said closed off. In this way, the corresponding gauges of the different zones are subjected to distinct stresses when bending the bar 4, but the combination of the gauges makes it possible to produce a substantially independent electrical signal.

  of the longitudinal eccentricity of the effort.

  In the embodiment shown in FIGS. 2 to 5, the flexion bar 4 carries four strain gauges arranged in the following manner: a first gauge 20 sensitive to variations in surface length of the flexion bar is fixed to the face upper 21 in the vicinity of the first end 7 of said bending bar 4. A second gauge 22 is fixed in the same area on the lower face 23 of the bending bar 4. A third gauge 24 is fixed on the upper face 21 of the bending bar in the vicinity of its second end 10, and a fourth gauge 25 is fixed in the same zone on the lower face 23 of the bending bar. The gauges 20, 22, 24 and 25 are sensitive to variations in length of the surface of the bending bar 4. Such cells sensitive to variations in length are well known in the art, and are shown diagrammatically in FIGS. 4 and 5 They comprise a series of resistive filaments arranged longitudinally and connected in series between two connection terminals. The strain gauges 20, 22, 24 and are electrically connected to form a Wheatstone bridge, as shown in FIG. 15, between two supply terminals 26 and 27 intended to be connected to a power supply source E, and between two measurement terminals 28 and 29 connected to a measurement device

e of the electrical signal.

  The variant shown in Figures 6 to 11 facilitates the wiring of the gauges. In this variant, the strain gauges are distributed in two zones 11 and 12, on the same upper face of the bending bar 4. Each zone 11 or 12 comprises two strain gauges. In the first embodiment shown in FIGS. 8 and 9, the strain gauges 20 and 24 are sensitive to variations in the surface length of the bending bar 4, while the strain gauges 22 and 25 are sensitive to variations in width surface of the bending bar 4. The gauges 20 and 22 are grouped in the surface area 11, and the gauges 24 and 25 are

  grouped in area 12 of the surface.

  In the embodiment of FIGS. 10 and 11, the four strain gauges 20, 22, 24 and 25 are sensitive to variations in the surface length of the bending bar 4. The gauges 20 and 25 are then grouped in zone 11 of surface, and we group the gauges 22

and 24 in area 12 of the surface.

  In the two embodiments, the strain gauges are connected as shown in FIG. 15, so that each bridge branch, such as the branch formed by the gauges 20 and 22 in series, comprises a positive variation resistance gauge and an

  gauge with negative variation of resistance.

  In the embodiment of FIGS. 12 to 14, the bending bar 4 comprises four strain gauges distributed in a single zone, of reduced surface, arranged in the vicinity of one or the other of the ends 7 and 10. For example , the four strain gauges are arranged in the zone 12 near the second end 10 of the bar

  bending 4. In this case, the offset of the longitudinal eccentricities

  loading is carried out by providing an angle A between the

  bending 4 and the secondary blade 13, which are then no longer parallel.

  For this, the first respective ends 7 and 14 of the bending bar 4 and of the secondary blade 13 are spaced from each other by a distance D1 greater than the distance D2 separating their second respective ends 10 and 15. The value of angle A can be

  determined experimentally, and depends on the geometric characteristics

  dimensions and dimensions of the elements forming the load cell, and

  particular of the chosen position of the zone of the strain gauges.

  This value is generally low, for example less than a degree.

  The compensation effect introduced by angle A can be explained by the fact that it introduces into the zone of the strain gauges, during the longitudinal compression or tensile forces of the bending bar 4, a bending deformation in opposite direction of deformation

due to buckling.

  In this embodiment of FIGS. 12 to 14, it is possible, for example, to arrange the strain gauges as shown in FIG. 14, with two strain gauges 22 and 24 sensitive to variations in the surface length of the bending bar 4, and two strain gauges 20 and 25 sensitive to variations in the surface width of

the bending bar 4.

  A similar embodiment can be provided by placing the four strain gauges in the zone 11 close to the first

  end 7 of the bending bar 4.

  It is understood that the embodiments of FIGS. 12 to 14 in which the four strain gauges are grouped in a single zone are advantageous, because the electrical connections are

greatly facilitated.

  In the embodiments which have been described, it is advantageous to use four strain gauges fixed on the bending bar 4 and connected in bridge. The arrangement of the 4 gauges on the bar reduces the disadvantages due to thermal drifts. It is however possible, without departing from the scope of the invention, to use only two strain gauges fixed on the same face of the bending bar and forming a first branch of the Wheatstone bridge, for example gauges 20 and 22, the other two gauges such as gauges 24 and 25 forming the second branch of the bridge then being replaced by electrical resistors of the same fixed values. This structure with two strain gauges and two resistances is applicable to each of the gauge arrangement modes described above and represented on

the figures.

  The present invention is not limited to the embodiments.

  which have been explicitly described, but it includes the various

  variants and generalizations contained in the field of claims

the following.

Claims (7)

  1 - Load cell with bending bar and strain gauges for force measurement in a weighing apparatus, comprising - an elastically deformable bending bar (4) arranged in a direction substantially perpendicular to the direction (5) of the force to be measured, a mobile upright (6) secured to a first end (7) of the bending bar (4) and generally parallel to the direction (5) of the force to be measured, intended to be secured to a plate (2) receiving the load to be measured (8), - a fixed upright (9) secured to the second end (10) of the bending bar (4) and generally parallel to the direction (5) of the force to measure, intended to be integral with a base (3), - strain gauges (20, 22, 24, 25), fixed on the surface of the bending bar (4), and intended to be electrically connected to a electrical measurement circuit to supply an electrical signal as a function of the bar's deflection re flexion (4) under the action of the mechanical force applied between the fixed (9) and mobile (6) uprights, characterized in that it further comprises a secondary blade (13) flat and wide, elastically deformable , disposed away from the bending bar (4) in a plane substantially perpendicular to the direction of the force to be measured (5), fixed respectively at its first end (14) to the mobile upright (6) and according to its second end (15) to the fixed upright (9), so that the secondary blade absorbs forces   torsional due to the eccentricity of the force to be measured.   2 - Cell according to claim 1, characterized in that - the movable upright (6) is integral with a load receiving plate (2) developing along a length L1 parallel to the bending bar (4) and along a width L2 perpendicular to the bending bar (4), the secondary blade (13) has a substantially constant width L   and close to the width L2 of the load receiving plate (2).   3 - Cell according to one of claims 1 or 2, characterized   in that, in the direction of the flexions produced by the force to be measured, the stiffness of the flexion bar (4) is much greater than the stiffness of the secondary blade (13).   4 - cell according to any one of claims 1 to 3,   l characterized in that the strain gauges (20, 22, 24, 25) are distributed in at least two zones (11, 12) longitudinally spaced apart from one another on the bending bar (4), at equal distances from the center of the bar, so that the gauges are subjected to separate stresses when bending the bar, and the   combination of gauges produces a sensitive electrical signal   ment independent of the longitudinal eccentricity of the force.   - Cell according to any one of claims 1 to 3, characterized in that -   - the strain gauges (20, 22, 24, 25) are arranged in a single zone (12) of the bending bar (4), away from the central zone, the respective first ends (7, 14) the bending bar (4) and the secondary blade (13) are spaced from each other by a distance D1 greater than the distance D2 separating their respective second ends (10, 15), so that the bending bar (4) made with the secondary blade (13) at an angle A, the value of which is adjusted so as to compensate for the effects of longitudinal decentering of the force.   6 - cell according to any one of claims 1 to 5,   characterized in that it comprises four strain gauges (20, 22,   24, 25) fixed on the bending bar (4).   7 - cell according to any one of claims 1 to 5,   characterized in that it comprises two strain gauges (20, 22) fixed on the bending bar (4) and associated with two resistors   electrical (24, 25) of the same fixed values.   8 - Libra, characterized in that it comprises   - a load cell (1) according to any one of claims 1   at 7, - a load receiving plate (2) secured to the movable upright (6) and arranged in a plane substantially parallel to the bending bar (4) and to the secondary blade (13), - a base (3) secured of the fixed upright (9) and arranged in a plane substantially parallel to the receiving plate (2), - an electrical circuit connecting the strain gauges according to a Wheatstone bridge (20, 22, 24, 25) between two supply terminals ( 26, 27) intended to be connected to an electrical power source (E) and between two measurement terminals (28, 29) connected to a   electrical signal measuring device. e